The invention relates to a material and to a process for thermal coating, to a surface layer of the material and also to a compressor with such a surface layer.
Ceramic coatings applied by thermal spraying have been known for a long time for a multitude of applications. Thus, by way of example, surfaces of oil lubricated cylinder running surfaces in vehicle engines have already been coated for some time by plasma spraying amongst other things, with the layer significantly reducing above all the coefficient of friction which is effective between the piston rings and the cylinder wall whereby the wear of the piston rings and the cylinder is significantly reduced, which leads to an increase of the working life of the engine, to a prolongation of the service intervals, for example between oil changes, and not least to a notable increase in the performance of the engine. This is achieved by different measures. For example such layers can contain deposits of dry lubricants in a base matrix for oil lubricated combustion engines, with it also being possible to provide pores of a predeterminable size in the base matrix which function as oil pockets and thus, together with the relatively soft embedded dry lubricants, significantly reduce the friction between the piston rings and the cylinder wall. The base matrix itself, which contains the dry lubricants and the pores amongst other elements, is built up of a hard matrix material which guarantees a long working life of the cylinder running surface and of the piston rings. A modern high performance cylinder running surface of this kind is for example described in detail in EP 1 340 834.
Further typical applications for surfaces applied by thermal spraying are the coating of turbine parts with wear protection layers and heat insulating layers, and the coating of components of oil lubricated bearings, such as for example the coating of crankshaft bearings or other workpieces exposed to special physical, chemical or thermal loadings. Depending on the purpose which the layer has to satisfy, quite specific materials are used, as a rule in the form of spray powders or spray wires which have the necessary specific characteristics and composition in order to generate the required characteristics of the surface layer to be sprayed.
Whereas the previously mentioned highly developed materials for the production of surface layers lead in the case of oil lubricated applications to excellent results in the technical application, these materials are however completely unsuited for the case in which the use of lubricants is to be avoided, since these materials were indeed specially developed for use in oil lubricated applications.
An important example for an application, which brings considerable disadvantages when using a lubricating liquid, is compressors for the compression of gases. Such compressors are well known in quite diverse embodiments, for example as rotary piston compressors or as reciprocating piston compressors. Reciprocating piston compressors in particular are widespread and have a high technical importance, for example as single-stage compressors when the pressures to be generated are not too high, or as multi-stage compressors, above all for high pressure applications.
In this connection they can be used for the compression of all possible gases, starting with customary environmental air, via pure oxygen, nitrogen, natural gas, noble gases, hydrogen or any other gas or gas mixture. It will be understood that the specific constructional and technical designs of the compressors used vary, depending on the gas which is to be compressed or the pressure range in which the compressed gases are to be made available. All these types of compressors have indeed been known in principle for a long time so that their special technical details do not need to be discussed further at this point.
In principle a reciprocating piston compressor essentially includes a cylinder in which a piston, quite similarly to a reciprocating piston combustion engine, is arranged to be movable to and fro for the compression of a gas. The compression chamber is bounded by the wall of the cylinder, by the piston movably arranged therein and by a cylinder cover. The movement of the piston in the cylinder is generated via a connecting rod connected to a crankshaft, with the crankshaft being driven for example by an engine, for example an electric motor, or by coupling to a combustion engine or to another drive unit.
When the piston is located in the vicinity of the lower dead center position, the gas to be compressed is introduced into the compression space and the gas can be compressed in the reducing volume of the compression chamber to a higher pressure during a subsequent compression stroke, in which the volume of the compression chamber is greatly reduced by a movement of the piston in the direction towards its upper dead center point. In the vicinity of the upper dead center point the gas which stands under the elevated pressure is then directed further via a valve for example into a further compression stage for a further pressure increase or, when the pressure is adequately high, can for example be supplied to a pressure container where it is then available for further use.
In order that a sufficiently high pressure can be produced during the compression stroke of the piston, that is to say during reduction of the volume of the compression chamber by the movement of the piston in the direction towards the upper dead center point, the piston must be sealed as well as possible relative to the cylinder wall in which the piston is executing its movement in the axial direction. For this purpose the piston as a rule has sealing rings formed as piston rings which are bedded in well-known manner in a groove which runs around the piston and stand under a certain radial pre-stress, so that the sealing ring is pressed with a specific force against the running surface of the cylinder wall. A sealing effect is hereby produced so that the gas enclosed to the cylinder can be compressed to a predeterminable pressure. A plurality of such sealing rings are mainly arranged in the form of a packing in one or more peripherally extending grooves at the piston, whereby the sealing action is increased. In this connection, arrangements with pistons without sealing rings are for example known for special applications, if no particularly high pressure is to be produced.
A relatively good seal can admittedly be achieved amongst other things by the previously described measures; however because considerable friction forces arise by the contact between the sealing ring and the running surface of the cylinder wall and/or by the contact between the piston and the running surface of the cylinder wall through the movement of the piston, these friction forces are to be minimized.
This preferably takes place by the use of a lubricant such as for example a lubricating oil, whereby the friction between the parts moved relative to one another can be adequately reduced. Moreover, the seal at the periphery of the piston can be additionally held by the lubricant.
Irrespective of which special application or embodiment of the compressor is used, considerable problems exist with these oil lubricated compressors known from the prior art due to the contamination of the gases to be compressed by the lubricant.
The problems as a result of contamination of the gases by the lubricant can be of different types. For example, if high purity gases are required, for example for laboratory use, or if compressed gases are present which are to be burned in highly complicated components of combustion systems, contamination of the system components by lubricating oil can arise. Thus, for example with vehicles operated with natural gas, components of the injection system such as the injection pump or the injection nozzles can be contaminated. For reasons of environmental protection a contamination of the compressed gases is frequently also highly undesired because lubricant can for example enter into the environment as a finely distributed mist or can be burned at the same time, for example in a vehicle operated with natural gas, whereby noxious combustion products can arise. In addition to contamination, which can lead to blockages or restrictions in fine high pressure lines amongst other things, problems arise not infrequently with enhanced corrosion, for example of components such as metallic high pressure lines, nozzles in the system or pump parts, since the lubricating oils can act aggressively on specific materials, physically, chemically or thermally and so bring about their premature wear.
A further serious problem is the formation of fine lubricant droplets in the compressed gas. Thus, for example, it can transpire that the lubricant distributed in the compressed gas condenses to the above-named droplets which then, on relaxation of the compressed gas, can turn into regular projectiles in a very fast gas flow which, for example, can strike walls or surfaces of attached system components and there cause massive damage as a result of their high kinetic energy.
Compressors for the compression of air for the actuation of brakes in vehicles are an example of known oil lubricated compressors which are very important in practice and which are confronted with essentially all these problems to a greater or lesser degree in operation. In this connection braking systems for rail vehicles, aircraft, motor cars or trucks should be named here in particular, but not exclusively. Such braking systems have indeed been known for a long time and are as a rule operated with compressed air which is made available in a pressure storage container. The pressure in the storage container is in this respect maintained or built up by a suitable compressor which can for example either have its own drive system, for example an electric motor, or can be coupled to the driving engine of the vehicle or driven in another manner.
In the above-named braking systems, in addition to increased wear, the operational security particularly plays a decisive roll, which can be impaired by contamination of the compressed air with lubricant and in the worst case can lead to a failure of the braking system.
It would thus be desirable to use a dry running compressor, that is to say, a compressor which does not require any lubricant between the sealing ring and the running surface of the cylinder wall of the compressor chamber, and/or between the compression piston and the running surface of the cylinder wall of the compression chamber, for a reliable operation of the compressor.
In principle, ceramic alumina coatings have indeed been known, by way of example, for a longer period of time as wear protecting layers or also as electrical insulators. In this connection, in order to reduce the brittleness of the alumina, TiO2 is for example added as an alloying element. In order to increase the toughness of the brittle alumina it is also known on the other hand to add ZrO2, so that through a conversion (from tetragonal to monoclinic) which is coupled with a volume change into a zirconium phase, the toughness relative to pure alumina is increased. The increase of the toughness is thereby related to compressive stresses which are produced in a sprayed-on coating by the conversion into the zirconia phase during cooling following the spraying process.
However, the improved toughness in the materials known from the prior art is always bought at the cost of the layers so that these are unsuitable, in particular in coatings for parts which are in rubbing contact in the dry state, i.e. not lubricated with lubricants or have a restricted working life. These materials are in particular very poorly suited as a coating in a dry running compressor because the reduced hardness rapidly leads to the formation of fretting wear so that the compression chamber is no longer adequately sealed after only a short period of operation. That is to say, a part of the air compressed in the compression chamber can escape through scores which, for example, form in the coating running surface of the compression cylinder as a result of the low hardness of the layer, so that the working pressure which is required can no longer be built up.
Layers whose toughness was not increased by the addition of further alloying components such as zirconia are at least just as unsuitable. Such layers have too little ductility and are thus too brittle, so that the layers rapidly lead to wear under mechanical and thermal loading, for example by the formation of fractures, cracks or scores in the surface.